Multilayer drawn glass fibers are of increasing importance for the transmission of light beams over long distances, expecially for light-wave communications. To avoid excessive light losses, it is now common practice to form composite drawn fibers having a glass core of one optical index of refraction and a surrounding cladding glass layer of a lower index of refraction. Single-mode fibers may have a core diameter of only a few microns and an outer diameter of core and cladding of from 10 to 100 times greater; whereas multi-mode fibers may have much larger core diameters, e.g., longer than 60 microns, up to 100 microns, and outer cladding diameters up to about 150 microns. The cladding layer is customarily enclosed by one or more layers of a suitable plastic to provide physical protection for the delicate fiber. Even so, problems of low fiber strength, inadequate fiber durability, and short life have remained.
When a tensile force or bending stress is applied to the fiber, tension in the outer fiber surface imcreases substantially. Even when precautions are taken to keep dust particles and moisture from the outer surface of the glass fiber structure, as by immediate application of the plastic coating during manufacture, the fiber is usually somewhat abraded and micro-cracks tend to form on the fiber surface. Since an optical communication fiber may be subject to considerable tensile and bending stresses, dividing employment, such microcracks propagate readily from the perimeter of the glass surface toward the glass core. In due time the entire fiber becomes substantially weakened and may fracture after a relatively short life that is totally inadequate for communication purposes. The presence of water molecules on the glass perimeter will also enhance crack propagation, increasing the chances of early failure.
One known method of increasing the strength of glass optical communication fibers is to provide surface compression at the cladding surface. Such a technique is discussed, for example, in a article in the Journal of the American Ceramic Society, December 1969, pages 661-664, by D. A. Krohn and A. R. Cooper, then of Case Western Reserve University. This article presents theoretical and experimental data to show that, if the cladding is selected to have a lower coefficient of thermal expansion than that of the core glass and if proper attention is paid to glass transition temperatures of the core and cladding, there is a good probability that compressive stresses can be developed to improve fiber strength.
It has also been previously proposed in general terms to strengthen a composite optical fiber by applying a second sheath over the cladding sheath which has a lower coefficient of thermal expansion of the cladding sheath or of the combination of core and cladding sheath. See for example the German Federal Republic Offenlegungsschrift 24 19 786, published Nov. 6, 1975. Reference may also be made to a corresponding English version in Australian Specification No. 493,505, published Oct. 21, 1976.
In a copending application Ser. No. 949,351, filed Oct. 6, 1978 by Charles K. Kao and Mokhtar S. Maklad, assigned to the same assignee as the present invention, new and improved three-layer and four-layer composite optical preforms and fibers are disclosed. By using fabrication techniques disclosed in this application, resultant highquality optical communication fibers can be produced having much higher surface compression and higher tensile strengths than those taught by the prior art as typified by the references cited above. Briefly, this is accomplished by fabricating the multilayer preforms and resultant fibers from various glass materials having carefully-selected glass compositions, thicknesses, coefficients of expansion and glass transition temperatures, as set out in detail in the Kao et all application. By these techniques, they have found it possible to obtain compressive stresses in the outer surface of the completed fiber of 50,000 pounds per square inch (50 kpsi). or higher.
The referenced copending Kao et al. application also discloses a method of manufacturing a preform for such a composite glass fiber in which selected glassy materials, which will later form the core and surrounding layers of the fiber, are deposited by chemical vapor deposition (CVD) techniques on the inner surface of a hollow tubular substrate of a silica material. The substrate and enclosed annular layers, which have been deposited in inverse order with a core layer deposited last, are then collapsed under increased heat into a solid preform structure. These methods and techniques, as thus far described, are well known in the art with variations thereof being described in U.S. Pat. Nos. 3,982,916; 4,009,014; and U.S. application Ser. No. 704,146 filed July 12, 1976 now U.S. Pat. No. 4,140,505 issued Feb. 20, 1979. However, before this preform is reheated and drawn out into an optical fiber, also by known techniques, Kao et al. employ further processing to obtain much higher compressive stresses on its outer surface than can be produced at the surface of the relatively-thick outer layer formed from the collapsed substrate tube. They disclose two methods for removing all, or substantially all, of the substrate layer employing either precision grinding off and polishing, or milling off this layer by a highintensity CO.sub.2 laser beam. This can be accomplished, as taught in their application, so as to leave a first, relatively thin material that was initially deposited in the substrate tube as the outer layer. They also teach how to make this layer a very high compression layer. From this completed preform a long optical fiber can then be drawn having greater durability, strength and fiber life than previously thought possible.
However, it is sometimes difficult, when using the above-described grinding or laser-milling techniques, to remove the substrate layer uniformly if the collapsed preform is not entirely straight or if the core and various layers are not precisely concentric. Furthermore, the desired outer layer of the completed preform and fiber must have a thickness of only a few microns to achieve the desired high compressive stress. For example, Kao et al. have determined that the radial thickness of the outer high-compression layer in the completed structure should be less than 10 microns, and preferably in the range of 1 to 5 microns. I have discovered how to remove the unwanted substrate material with even greater precision than previously attained, regardless of nonlinearity or lack of concentricity in the collapsed preform, by using a novel combination of materials and processing techniques to be described below.